We have an approximately 1 acre pond that doesn't stay full, but does stay consistently at about the same level. We can get a good rain that fills it up, but it will drop 3-4 feet over the next weeks. The pond has a spring that nearly always feeds it from the north. The most curious matter though is the fact that jets of air stream up from fissures in the rock after a large rain. I swam down and located the source of the air, then attempted to pack one of the fissures with bentonite, but the air just came out further down the crack. Erosion has been an issue around the pond, revealing lots of the bedrock. As I mention in the video post I linked to, we recently had a sinkhole entrance open up in our pasture, but I don't know the extent of the underground geologic features that may contribute to this phenomenon. The spring that feeds the pond comes out of the hill maybe 3 feet above the sub-full level the pond stays at. Have any of you seen something similar? The only thing I can figure is there is an air filled cavern that connects to the pond via the fissures in the rock. When the water rises, the weight of it displaces the air in the caver, pushing it out through the cracks. I would really like to get the pond to hold maximal water so I can stock it accordingly, but I am concerned that the water may go both ways through the cracks--into and out of the cavern I've theorized is there--which makes me think anything I plug the leaks with will be pushed up and away from the hole(s) just as much as the water pressure presses it down to seal. I'm not familiar with image/video hosting options, so I've linked to my personal FB page where I posted a video you can reference. Any insight would be greatly appreciated. [img]https://www.facebook.com/aj.allison.75/videos/1220699855596015[/img]
Your analysis of the water/air behavior in your cavern sounds spot on to me.
I do not believe bentonite will seal sizeable fractures/crevices in bedrock.
Normally, we use a clay blanket with a thickness of 12" to seal a pond that is 10 feet deep. That thickness is sufficient to stop the hydrostatic pressure of 10 feet of water depth trying to go to atmosphere on low ground outside of the pond. Your situation is just the opposite of that.
How much elevation change is there from your pond relative to the highest surrounding hills? Water that enters that cave-aquifer during a big rain event could potentially have that much hydrostatic pressure trying to push out any sealing material that you put into the bedrock crevices in your pond basin.
Fortunately, that is the potential MAXIMUM pressure. Your actual required sealing pressure should be lower. You just need a seal that continues to re-route water in the aquifer past the point of restriction around your pond and send the water further down the slope of the aquifer. Unless your pond basin is the ONLY route for the cavern system (unlikely), then hopefully you can construct a seal with sufficient resistance to the hydrostatic pressure of the aquifer in your immediate vicinity.
I do not see any quick and inexpensive fixes to your problem. (If we have diagnosed it correctly.) Do you have clay available near your pond? One option might be to excavate the entire bottom of your pond again, and expose all of the bedrock. Remove all of the boulders and cobbles that can be removed to the load rating of your equipment. Pull any boulders too heavy for removal away from your bedrock exposures. Bring in clay, wet it to the optimal compaction water content, and pack in your pond bottom in 6" lifts. Maybe try a blanket of 2 feet total thickness over your exposed rock?
Sorry about your leaky pond. Hopefully you can devise some solutions!
You might be able to drain it and apply a new clay liner with some success, but there are no guarantees that it will hold. Installing a synthetic liner might be a better bet.
I know of a few problem ponds located on the karst near Rock Bridge State Park south of Columbia. One in particular is noted for dumping its contents into the cave system every ten or twelve years.
